No reason to scare them to surrendering. The Japanese had very little military hardware left by this time and the Russians were going to declare war on the Japanese on Aug. 8 (2 days after the bomb was dropped) which would have been devastating to the Japanese and probably would have prompted their surrender. In my opinion Truman dropped the bomb, against strong military advice not to use the bomb, to send a message to the Russians and to protect US present and future Asian markets. If the Russians would have entered the war and become part of the Pacific alliance then they would have received a piece of the plunder which is something that the US could not have because of the open door policy. The US was looking for Asian markets and had been very active during the war acquiring them. The Russians, having a closed market, would have been counter intuitive to US foreign policies.
So, no, the bombs were not necessary.